Photoactivatable caged compounds with AIE characteristics: method of preparation and applications

Abstract

A photoactivatable caged compound (TPE-C) with AIE characteristics is designed and synthesized. TPE-C is non-emissive either in solution or in aggregated state, but its luminescence can be induced to emit strong cyan emission in aggregated state by UV irradiation. Such property enables TPE-C to be applied in photo-patterning and anti-counterfeiting related areas.

Claims

1. A caged compound, comprising an AIE compound as a luminophore and a 2-nitrobenzyl group as a quencher, wherein the 2-nitrobenzyl group is ##STR00018## wherein the caged compound is selected from the group consisting of ##STR00019## ##STR00020## wherein R, R, R or R are independently selected from the group consisting of H, ##STR00021## with the proviso that the caged compound comprises at least one ##STR00022##

2. The caged compound of claim 1, wherein the caged compound is TPE-C ##STR00023##

3. A process for preparing a caged compound of claim 1, wherein the process comprises reacting a compound selected from the group consisting of compounds of formula I, formula II, formula III, formula IV, formula V, formula VI, formula VII, formula VIII, and formula IX with 2-nitrobenzyl bromide ##STR00024## ##STR00025## wherein R, R, R or R are selected from the group consisting of H, ##STR00026## and at least one of R, R, R or R is ##STR00027##

4. The process of claim 3, comprising reacting TPE-P with 2-nitrobenzyl bromide (compound 3) to form TPE-C ##STR00028##

5. The process of claim 4, wherein the compound TPE-P is prepared by reacting 4-bromotetraphenylethene (compound 1, TPE-Br) with (4-hydroxyphenyl) boronic acid (compound 2), ##STR00029## ##STR00030##

6. A photoactivation process of a caged compound of claim 1, comprising exposing the caged compound of claim 1 to lights.

7. The photoactivation process of claim 6, wherein the caged compound of claim 1 is in an aggregated or solid state.

8. A method of photo-patterning, comprising: loading a caged compound of claim 1 on a substrate; placing a mask over the substrate to form a masked substrate, wherein a portion of the mask is transparent to a UV light; irradiating the masked substrate using the UV light so that the UV light passes through the portion of the mask transparent to the UV light and activates the caged compound of claim 1 underneath the mask, wherein the activated caged compound emits light to form a pattern on the substrate.

9. The process of claim 4, wherein the compound TPE-C is prepared by the following steps: ##STR00031## wherein TPE-P is synthesized via Suzuki coupling between 4-bromotetraphenylethene (1) and (4-hydroxyphenyl) boronic acid (2), and the TPE-P further reacts with 2-nitrobenzyl bromide (3) in the presence of Cs.sub.2CO.sub.3 to obtain TPE-C.

10. The process of claim 3, wherein compounds of formula I, formula II, formula III, formula IV, formula V, formula VI, formula VII, formula VIII, and formula IX respectively are prepared by reacting compounds of formula I, formula II, formula III, formula IV, formula V, formula VI, formula VII, formula VIII, and formula IX with ##STR00032## ##STR00033## ##STR00034## wherein R, R, R or R are selected from the group consisting of H, ##STR00035## and Br, and at least one of R, R, R or R is Br.

11. The method of claim 8, further comprising: removing the mask from the substrate; and irradiating the substrate without the mask so that substantially all the caged compound of claim 1 on the substrate is activated.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Various embodiments will be described in detail with reference to the accompanying drawing.

(2) FIG. 1 Uncaging process of TPE-C.

(3) FIG. 2 Synthetic route to TPE-P and TPE-C.

(4) FIG. 3 .sup.1H NMR spectrum of TPE-C in CD.sub.2Cl.sub.2.

(5) FIG. 4 .sup.13C NMR spectrum of TPE-C in CD.sub.2Cl.sub.2.

(6) FIG. 5 High resolution mass spectrum (MALDI-TOF) of TPE-C.

(7) FIG. 6 Absorption spectra of TPE-P and TPE-C in THF solution. Concentration: 10 M.

(8) FIG. 7(A) PL spectra of TPE-P in THF/water mixture with different water fractions (f.sub.w). (B) Plot of relative PL intensities (I/I.sub.0) versus f.sub.w. I.sub.0 are the PL intensities at 488 nm of the dyes in THF solutions; Dye concentration: 10 M; excitation wavelength: 320 nm. Inset: photographs of (a) TPE-P and (b) TPE-C in f.sub.w=95% excited by hand-held UV lamp at 365 nm.

(9) FIG. 8 Particle size analysis of TPE-P (A) and TPE-C (B) in the mixture of THF/water with f.sub.w=95 vol %.

(10) FIG. 9 Proposed uncaging mechanism of TPE-C.

(11) FIG. 10(A) PL spectra of TPE-C in 95% f.sub.w with different irradiation time. (B) Change in PL intensities at 488 nm of TPE-C in different f.sub.w versus irradiation time. Concentration: 10 M; excitation wavelength: 320 nm. Inset: photographs of TPE-C in 95% f.sub.w excited by hand-held UV lamp at 365 nm under UV irradiation for 0 s and 240 s.

(12) FIG. 11 Fluorescent photographs of TPE-C in the mixture of THF/water with f.sub.w=95 vol % taken at different irradiation time.

(13) FIG. 12 HPLC spectra of TPE-P (A) and TPE-C before (B), and after being irradiated under UV light for 3 (C) and 8 (D) min respectively. All samples were eluted with acetonitrile at a flow rate of 1 mL/min.

(14) FIG. 13 Change in the integrated area of the specific peaks in HPLC spectra versus irradiation time.

(15) FIG. 14 High resolution mass spectrum (MALDI-TOF) of the peak at 1.5 min in HPLC spectrum.

(16) FIG. 15 Photographs of filter paper at ambient conditions taken under UV irradiation at 365 nm with different irradiation time. Letters A and E were written by TPE-P and letter I was written by TPE-C using capillary tubes.

(17) FIG. 16 Photographs of the process of (A and C) photo-pattern by a mask with HKUST logo under UV irradiation and (B and D) pattern erasing process after removing the mask under further UV irradiation.

(18) FIG. 17 Photographs of the process of photo-pattern by a mask with HKSAR logo under UV irradiation and pattern erasing process after removing the mask under further UV irradiation.

DETAILS DESCRIPTION OF THE INVENTION

(19) The present invention can be illustrated in further detail by the following examples. However, it should be noted that the scope of the present invention is not limited to the examples. They shoud be considered as merely being illustrative and representative for the present invention.

(20) The synthetic route of TPE-C is depicted in FIG. 2. TPE-P is synthesized via Suzuki coupling between 4-bromotetraphenylethene (1) and (4-hydroxyphenyl) boronic acid (2). The resultant TPE-P is then reacted with 2-nitrobenzyl bromide (3) in the presence of Cs.sub.2CO.sub.3 to furnish TPE-C. The product was characterized by NMR and mass spectroscopy and both of them gave satisfactory analysis data corresponding to their molecular structure (FIG. 3-5).

(21) The UV spectra of both TPE-C and TPE-P in THF solution exhibit an absorption maximum at 320 nm (FIG. 6). As shown in FIG. 7, both TPE-C and TPE-P are non-fluorescent in pure THF solution. The emission of TPE-P increases swiftly when the water fraction (f.sub.w) in the mixture of THF/water exceed 85%. When f.sub.w=95%, the PL intensity at 488 nm is 80-fold higher than that in pure THF solution. The fluorescent enhancement of TPE-P is attributed to the formation of nanoaggregates (FIG. 8), suggesting that TPE-P is AIE-active. On the other hand, TPE-C remains non-emissive although the nanoaggregates have formed when 95% of water is added to the THF solution. The PL results indicated that the emission of TPE-C in aggregated state is quenched by 2-nitrobenzene through PeT process. Inspired by the uncaging process in conventional systems, TPE-C is expected to respond to UV irradiation. As the proposed uncaging mechanism shown in FIG. 9, the 2-nitrobenzyl group in TPE-C is cleaved by UV irradiation and TPE-P and 2-nitrosobenzaldehyde are readily formed. Since TPE-P is highly emissive in aggregated state, we utilized PL measurements to monitor the uncaging process of TPE-C in aggregated state. As shown in FIG. 10, TPE-C shows a weak emission in both the mixtures of THF/water with f.sub.w=85% and 95%. Their PL peak intensities at 488 nm are gradually enhanced along with the UV irradiation time, indicating that the TPE-P is formed during UV treatment (FIG. 10A). The PL enhancement in f.sub.w=95% is faster than of f.sub.w=85%, it should be because the TPE-P aggregates in 95% water content are more compressed and the intramolecular motions are more restricted (FIG. 10B). The fluorescent photos of TPE-C in the mixture of THF/water with f.sub.w=95% taken under UV illumination also demonstrate such an uncaging process (FIG. 11).

(22) To further verify whether the cyan fluorescence is attributed to the formation of TPE-P, we conducted high-performance liquid chromatography (HPLC) to monitor the uncaging process. We first run the pure TPE-P and TPE-C using acetonitrile as the references. The peaks for TPE-P and TPE-C are observed at 1.5 and 2.0 min, respectively (FIG. 12). The TPE-C aggregates in the mixture of THF/water with f.sub.w=95% are then irradiated by UV light and the samples are taken out for HPLC analysis in every minute. The HPLC spectra show that the peak area for TPE-C is decreasing while the peak area for TPE-P is increasing along with the UV irradiation time (FIG. 13), which is consistent with the PL results. As indicated by the mass analysis, the isolated product from HPLC at 1.5 min has an exact mass of 424.1822 (FIG. 14), which corresponds to the mass of TPE-P.

(23) Synthesis of TPE-P.

(24) Into a 250 mL two-necked round bottom flask equipped a condenser. 4-Bromotetraphenylethlene (2.00 g, 4.86 mmol), (4-hydroxylphenyl)boronic acid (2, 0.74 g, 5.35 mmol), sodium bicarbonate (5.14 g, 48.62 mmol) and Pd(PPh.sub.3).sub.4 (0.17 g, 0.15 mmol) were dissolved in to 90 mL distilled THF and 30 mL water under nitrogen. The mixture was heated to reflux overnight. After being cooled to room temperature, the mixture was extracted with dichloromethane for three times. The organic phase was combined and washed with water and dried over anhydrous sodium sulfate. After the evaporation of solvents, the crude product was purified by silica gel column chromatography using DCM/hexane in the volume ratio of 1:3 as eluent. The white solid was obtained in the yield of 78%. .sup.1H NMR (400 MHz, CDCl.sub.3): 7.42 (dd, 2H, J=8.8 Hz), 7.28 (dd, 2H, J=6.8 Hz), 7.13-7.02 (m, 17H), 6.84 (dd, 2H, J=8.8 Hz). .sup.13C NMR (100 MHz, CDCl.sub.3): 154.3, 143.2, 141.5, 140.3, 139.9, 137.7, 132.8, 131.1, 130.8, 130.7, 127.5, 127.1, 127.1, 127.0, 125.8, 125.8, 125.1. .sup.1H NMR (400 MHz, CD.sub.2Cl.sub.2): 7.46 (dd, 2H, J=8.8 Hz), 7.33 (dd, 2H, J=8.4 Hz), 7.16-7.05 (m, 17H), 6.88 (dd, 2H, J=8.4 Hz). .sup.13C NMR (100 MHz, CD.sub.2Cl.sub.2): 154.7, 143.2, 141.5, 140.4, 140.0, 137.7, 132.4, 131.0, 130.6, 130.5, 127.3, 127.1, 127.0, 125.8, 125.7, 124.9. HRMS (MALDI-TOF) m/z 424.1821 (Mt, calcd. 424.5324).

(25) Synthesis of TPE-C.

(26) Into a two-necked round bottom flask, TPE-P (0.20 g, 0.47 mmol), 2-nitrobenzyl bromide (3, 0.12 g, 0.57 mmol) and cesium carbonate (0.18 g, 0.57 mmol) were dissolved in 7 mL acetonitrile under nitrogen atmosphere. The mixture was heated at 70 C. overnight. After being cooled to room temperature, the mixture was extracted with dichloromethane for three times. The organic phase was combined and washed with water and dried over anhydrous sodium sulfate Na.sub.2SO.sub.4. After the evaporation of solvents, the crude product was purified by silica gel column chromatography using DCM/hexane in the volume ratio of 1:4 as eluent. The pale yellow solid was obtained in the yield of 70%. 1H NMR (400 MHz, CD.sub.2Cl.sub.2): 8.14 (dd, 1H, J=8.0 Hz), 7.88 (dd, 1H, J=7.6 Hz), 7.69 (t, 1H, J=7.6 Hz), 7.15-7.48 (m, 3H), 7.32 (dd, 2H, J=8.0 Hz), 7.13-7.01 (m, 19H), 5.49 (s, 2H). 13C NMR (100 MHz, CD.sub.2Cl.sub.2): 157.1, 146.5, 143.2, 141.7, 140.4, 137.6, 133.3, 133.1, 131.0, 130.6, 130.5, 128.0, 127.8, 127.3, 127.09, 127.0, 125.8, 125.7, 125.0, 124.3, 114.5. HRMS (MALDI-TOF) ink 559.2153 (Mt, calcd. 559.6925).

(27) Application of TPE-C

(28) Inspired by rapid and highly efficient release of TPE-P from the caged compound TPE-C in aggregated state, we explored the possibility to utilize TPE-C as a kind of UV activatable fluorescent material for photo-patterning and anti-counterfeiting related applications. First of all, we tried to use filter paper as a substrate for writing. As shown in FIG. 15, the letter I is written with TPE-C while the letters A and E are written with TPE-P for comparison. Before UV treatment, the letters A and E are highly emissive but the letter I is still non-fluorescent. As the increase of UV irradiation time, the emission of letter I becomes stronger. Although the emission of letter I is still weaker than the letters A and E, it is understandable that the photoactivation process may only occur on the surface of filter paper and most of the TPE-C has not been uncaged.

(29) In addition to the fluorescent writing, TPE-C possesses the potential to be used in anti-counterfeiting applications. Since we have demonstrated that the photo-activation can be carried out on the filter paper, we can conveniently fabricate patterns or erase patterns by adding or removing a mask. Filter papers are firstly soaked with the THF solution of TPE-C and dried by compressed air. Two projector films with HKUST logo, one is transparent image (FIG. 16A) while the other one is dark image (FIG. 16C), were covered onto the filter papers. The HKUST logo gradually emerged on the filter papers upon UV irradiation. For the film with transparent logo, the frame structure displays brighter emission than the surrounding. On the contrary, the frame structure shows dimmer emission than the surrounding when a mask with a dark logo is used. Moreover, the patterns can be erased by further UV irradiation after removing the masks (FIGS. 16B and 16D). Since the caged fluorophore in both logo and surrounding areas are activated, the whole filter papers are emissive and the patterns cannot be seen as a resulted. To demonstrate the flexibility of this method, we used other films with different logo to perform the same experiment (FIG. 17). All the frame structures of patterns can be presented and also be erased. These photo-patterning and pattern erasing techniques can be potentially applied for one-time anti-counterfeiting protection, especially for high-valued products. We have designed and synthesized a new caged fluorophore based on a TPE derivative and a 2-nitrobenzyl group. The caged compound can be photoactivated and induced to emit strong cyan fluorescence in the aggregated state or solid state by UV irradiation. This property of the caged fluorophore enables it to be applied in photo-patterning and anti-counterfeiting related applications.